The invention relates generally to power-driven conveyors and methods for driving such conveyors. More particularly, the invention relates to belt or chain conveyors positively engaged by drive elements rotated by variable-speed drive motors.
Modular plastic conveyor belts and chains are constructed of rows of belt modules or chain links connected together by hinge pins at hinge joints into endless loops. Such a belt or chain is trained about rotatable drive and idler wheels, such as sprockets or drums. The drive wheels are rotated by motors. Control surfaces, such as teeth, on the periphery of the drive and idler wheels positively engage corresponding surfaces on the belts or chains. Because the hingedly joined chain links or belt rows are rigid, a chain or belt forms a polygon about the drive wheel. Thus, the radius of the belt or chain varies periodically around the drive wheel. As the motor rotates the drive wheel at a constant angular speed, the varying radius causes the linear speed of the belt or chain to fluctuate and the belt rows or chain links to rise and fall with respect to their line of engagement with the drive wheel. This “chordal action,” or “polygon effect,” causes a periodic vibrating motion and linear belt or chain speed variation, which can jostle conveyed articles in a product conveyor, annoy passengers on a moving walkway, or even induce resonance and high amplitude oscillations. The effect can be decreased by increasing the number of contact surfaces around the circumference of the drive wheel. FIGS. 1A and 1B show the linear speed of a belt or chain for 8- and 24-tooth sprockets driven at a constant angular speed. But, even with the smaller belt speed variation with 24-tooth sprockets, the effect may still be intolerable in certain applications. And the increased radius of a 24-tooth sprocket may not be acceptable in space-limited applications.
One approach that is used to decrease the vibration due to chordal action is to position guides at the entry of the belt to the sprocket to confine the range of the rise and fall of the belt. But impacts and rubbing between belt and guides can damage both. Another approach to reducing the chordal action is described in U.S. Pat. No. 6,892,874 to Pietz. The patent describes a device for reducing the polygon effect in a pedestrian conveyor. The device includes a reversing wheel, a gear acting on the reversing wheel, a power supply unit, and a variable-speed electric motor coupled to the gear and the power supply unit. The drive also includes a function generator, a controlling apparatus coupled to the function generator, and at least one position sensor for detecting a phase position of the reversing wheel. The sensor or sensors send the phase position to the controlling apparatus, which transmits a synchronized set speed value to the power supply unit based on the phase position. In this closed-loop control device, it is important for the controlling apparatus to maintain synchronism between the position sensor signal and the set speed signal. Furthermore, the device does not address the problem of resonance caused by the periodic speed variations.
Thus, there is a need for a belt conveyor in which the belt advances linearly at a constant speed, even with small-diameter drive wheels.